In order to detect and examine the presence of radioactive aerosols passed into the atmosphere, the method is known on how to monitor this presence through a filter on which the aerosol particles are retained and thus analyse the filtrate collected in front of specialized counting devices. These radioactive particles disintegrate, thus producing either alpha and beta (electrons) rays which are solid fragments, or X electromagnetic and gamma photons.
The spectrums obtained with the alpha particles have energies generally situated above 3 MeV and thus are easily differentiated from beta spectrums and the "Compton distribution" of photonic spectrums which, on the other hand, are situated in a given field of energy of several tens of KeV within 3 MeV. The alpha spectrums in addition have the advantageous property of comprising as regards the high energies a fast descent front which is almost vertical in energy/amplitude coordinates (FIG. 1).
However, the beta and photonic spectrums coinciding with energy are more difficult to separate.
The technique currently used to separately record the beta particles and the photons (X, gamma) consists of recording the sum of the beta+ spectrums (X, gamma) and then of inserting screens between the radioactive deposit to be measured and the detector, these screens stopping the beta particles; thus, the sole contribution due to the photons (X, gamma) is determined. A simple subtraction of the spectrums (X, gamma) from the sum of the beta + (X, gamma) spectrums makes it possible to know the spectrum of solely the beta particles.
In order that the result of this subtraction has the best possible significance, it is essential that the "responses" to the various radiations of the two detection chains are the same. This lays down two conditions, namely the identity of the chains and their energy-adjustment.
In order to achieve this, it is current practice to record the various beta and (X, gamma) spectrums with the aid of two strictly identical detection chains, the semiconductor detectors being placed extremely close to each other, one being used for overall beta+ (X, gamma) detection and the other provided with a screen absorbing the betas being used to solely detect the photons (X, gamma).
Another condition required to accurately know the contribution of the spectrum (X, gamma) within the overall spectrum and thus so as to accurately determine the beta spectrum(s) is the identical accurate adjustment in energy of the two counting chains.
According to the prior art, energy-calibration is effected with a standard beta source and a standard gamma source by means of the "yield method" which, for each of the standard gamma or beta sources used, consists of recording inside a specific geometry and for a defined energy window width and position a specific number of pulses per unit of time. By adjusting the gain of the counting chain, re-adjustment takes place on the defined "energy window".
But this calibration is only valid for the energy corresponding to that of the standard source. Secondly, these "windows" have a certain width, thus leading to a certain inaccuracy concerning the energy-adjustment.
The energy-adjustment of the invention is able to fully suppress these inaccuracies by using simple effective means.